Medals and coins (hereinafter representatively referred to simply as coins) sometimes have ridges as shown in FIG. 3 on the outer peripheral side thereof. The ridges are formed with the aim of decoration and differentiation from other coins.
For example, in the typical ridges found in 100 yen coins distributed as of 1997 in Japan, the direction of the ridge line ml of respective convex parts (hereinafter ridges) is, as shown in a simplified manner in FIG. 3(a), identical with the direction of rotation axis Y of a disc product, and so is the line of the notched bottom of the milled edge.
One example of ridges is a helical one. In this embodiment, the ridge line m2 forms an angle with the direction of rotation axis Y of a disc product, as shown in FIG. 3(b). The helical ridges have decorative functions and enable easy identification of a product due to the characteristic appearance.
As mentioned above, typical ridges have a ridge line that coincides in direction with the rotation axis of a coin, so that a single die may be used to forge both the relief plane and the ridges on the outer peripheral side. Then, the end product may be easily pushed out (knocked out) from the die with a punch that pushes the coin in the direction of the rotation axis. Thus, a coin having typical ridges permits use of a simple die and can be produced in large numbers at low costs.
In a coin having helical ridges, in contrast, the direction of the ridge differs from that of the rotation axis of the coin. Thus, pushing out by a simple movement of a punch as achieved with disc products with typical ridges is difficult to achieve. This constitutes a first obstacle.
What is more, a coin having helical ridges experiences difficulty in pushing out caused by a relief peculiar to coins. That is, a relief comprising letters and drawings depicted on the disc plane of the coin should be free of scars and nicks, and the coin should be pushed out without impairing the good appearance of this plane. For this end, the relief plane of the coin in a collar should be kept in continued close contact with the relief die on the punch. In this way, the friction between the relief plane and the relief die can be avoided during the time period from forging of the relief plane of the coin to pushing out thereof. Therefore, the movement of the coin in the collar during pushing out is limited in orders to maintain the quality of the relief. While the existence of a relief does not become an obstacle with respect to the pushing out for typical ridges, it constitutes a second obstacle for the pushing out of a coin having helical ridges.
When a coin having helical ridges is produced as mentioned above, it is difficult to forge and produce a coin having helical ridges and a relief in a single process from a material mass using a die having a simple structure. Thus, helical ridges have been formed in a different step such as rolling.
It is therefore an object of the present invention to provide a method for producing a disc product having a plane with a relief and helical ridges on the outer peripheral side, easily by die forging.
It is another object to provide, as a production apparatus, a forging die having a simple structure for the production of such a disc product.